Copper Ore-Modified Activated Coke: Highly Efficient and

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Cite This: Ind. Eng. Chem. Res. 2018, 57, 15731−15739

Copper Ore-Modified Activated Coke: Highly Efficient and Regenerable Catalysts for the Removal of SO2 Jin Yuan,† Xia Jiang,*,†,‡ Minjie Zou,† Lu Yao,†,‡ Chuanjun Zhang,† and Wenju Jiang†,‡ †

College of Architecture and Environment, Sichuan University, Chengdu 610065, China National Engineering Research Center for Flue Gas Desulfurization, Sichuan University, Chengdu 610065, China

‡

Ind. Eng. Chem. Res. 2018.57:15731-15739. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/22/18. For personal use only.

S Supporting Information *

ABSTRACT: A superior desulfurization catalyst (AC-Cop) was synthesized by blending copper ore and coals through a one-step carbonization−activation process. The sulfur capacities of the modified catalysts were enhanced significantly and that of AC-Cop with 1 wt % of copper ore was 174.8 mg/g, which was 50% higher than that of blank activated coke. Their excellent desulfurization performance could be attributed to the introduction of much more active sites on the surface of ACCop, i.e., CuO, Fe2O3, Fe3O4, and CO groups, which promoted the adsorption and oxidation of SO2. In addition, the used AC-Cop could be easily regenerated by thermal treatment, with a slow decrease in desulfurization performance during four desulfurization−regeneration cycles. This was ascribed to the gradual accumulation of the desulfurization products, i.e., Fe2(SO4)3 and CuSO4 and the decreased contents of CO groups. The results suggested that such copper ore-modified activated coke could be a kind of efficient, regenerable, and inexpensive desulfurization catalyst. Fe2O3, etc.7,8,14 Second, the common used impregnation method is laborious due to its complex post-treatment steps and moreover the special precaution is required owing to using caustic materials for the impregnation of AC.15 Third, it is difficult for the transition metals modified ACs to be regenerated because the active metals components were supported only on the surface of carbon materials, which is easily lost from the carbon matrix during the thermal regeneration process.16 Hence, it is very important to prepare a kind of new efficient modified AC with low cost and simple preparation process. Copper ore is widely distributed around the world, which contains a plenty of transition metals, such as Cu and Fe. If copper ore could be applied as the replacement of chemical agents for modifying AC, the production cost would be greatly reduced. Moreover, our previous studies have shown that blending was an extremely simple method, which needs relatively low cost, in comparison with the commonly used impregnation method.10,12,16 It is also highly possible that copper ore can react with carbon matrix during carbonization and activation process, which may help to modify surface chemical properties and porous structure of AC, resulting in high desulfurization activity. Meanwhile, the multiple metals in copper ore (i.e., Cu and Fe) might have synergistic effects on the removal of SO2, which would result in high desulfurization

1. INTRODUCTION Fine particulate matter (PM2.5, defined as particulates with aerodynamic diameter of 2.5 μm or less) is one of the major pollutants and has caused the most serious environmental issue. In China, more than 80% people lived in the regions where PM2.5 did not meet the air quality standard (35 μg/ m3).1 It was reported that sulfates accounted for up to 30% of PM2.5 mass, mainly resulting from the emission of SO2 during industrial process.2 The control of SO2 emission is the efficient source control method for PM2.5.3 Among various desulfurization methods, using activated coke (AC) to remove SO2 received ever-increasing scientific and industrial interest due to its small covering area, low water and energy consumption, no secondary pollution, and recovery of concentrated SO2 as useful sulfur-containing products.4−6 The key of AC desulfurization technology is to design a kind of highly efficient desulfurizer with low cost. It has been suggested that the desulfurization activity of AC could be significantly enhanced via the introduction of some transition metals, such as Fe,7 Cu,8 Ti,9,10 and Mn.11,12 Tseng et al.8 found that Cu species modification could improve the desulfurization performance of AC, owing to the catalytic effect of CuO on SO2 oxidation. When Fe species were supported on AC, the SO2 capture capacity increased obviously because of the formation of Fe-containing sulfates.7 In spite of their high sulfur capacity, transition metals modified ACs still have a low market share, compared with untreated one.13 This could be related to their several drawbacks. First, a high cost is needed for preparing modified AC due to the abundant use of pure chemical agents, such as Cu(NO3)2, Fe(NO3)3, CuO, and © 2018 American Chemical Society

Received: Revised: Accepted: Published: 15731

August 14, 2018 October 18, 2018 October 29, 2018 October 29, 2018 DOI: 10.1021/acs.iecr.8b03872 Ind. Eng. Chem. Res. 2018, 57, 15731−15739

Article

Industrial & Engineering Chemistry Research

Figure 1. SEM image (a) and EDAX results of AC-Cop1 (Cu (b) and Fe (c)).

X-ray fluorescence analysis (XRF-1800, Japan) was used to decide the chemical composition of copper ore. N 2 adsorption/desorption isotherms were carried out using a surface area analyzer (ASAP 2460, Micromeritics, USA) at 77 K. The specific surface areas (SBET) of the catalysts were obtained using BET equation. Micropore volume (Vmic) and mesopore volume (Vmeso) were calculated from the t-plot method and the BJH method, respectively. Total pore volume (Vtot) was calculated from adsorption amount at the maximum relative pressure. The morphology and the distribution of Cu and Fe on the surface of samples were observed under scanning electron microscopy (SEM, JEOL 7100F) with energy dispersive X-ray analysis (EDAX). The X-ray diffraction (XRD) patterns were acquired on an X-pert PRO MPD diffractometer employing Cu Kα radiation (λ = 1.5406 Å) over 2θ range 10−80°. Fourier transform infrared (FTIR) spectra of samples were measured in 4000−400 cm−1 region with a spectrometer (Nicolet 6700, Thermo, USA). X-ray photoelectron spectroscopy (XPS) was carried out to determine the surface chemistry properties of the samples, using a spectrometer (XSAM−800, Kratos Co., UK) with Al Kα radiation source. 2.4. Desulfurization and Regeneration. Desulfurization tests were carried out at 80 °C to evaluate the desulfurization activity of the prepared catalysts. The samples (20 ± 0.1 g) were packed into a glass column reactor (21 mm diameter, 100 mm length). Humidified gas (relative humidity at 80%) containing 3000 ppmv of H2S, 10% of O2, and balanced with N2 were passed through the reactor at a flow rate of 1200 mL/ min. A flue gas analyzer (Gasboard-3000, China) with a detection limit of 1 ppmv was applied to continuously monitor the outlet concentrations of SO2. The desulfurization test was stopped until the outlet SO2 concentration reached 300 ppmv, and the corresponding working time was regarded as breakthrough time. The sulfur capacity (mg SO2/g AC) of the sample was then calculated by integrating the SO2 breakthrough curve. After desulfurization test, thermal regeneration was carried out at 400 °C for 60 min under nitrogen condition in a tub furnace. The regenerated AC-Cop was denoted as “AC-CopRn”, where “n” represented the number of desulfurization− regeneration runs.

performance, compared with single metal.17 Such copper oremodified AC (AC-Cop) would provide a suitable and inexpensive solution for SO2 removal, overcoming the limitation of traditional activated coke desulfurizers, such as high cost, low sulfur capacity, and complex modification process. However, very few studies have been found on preparing copper ore-modified AC as desulfurizers until now. Therefore, a novel AC-Cop was designed and prepared by blending copper ore and coals through one-step carbonization−activation process for the removal of SO2. Furthermore, the porous structure and surface chemical property of the prepared catalysts before and after desulfurization was investigated to explore the desulfurization mechanisms of the AC-Cop. Finally, the thermal regeneration behavior of the ACCop was also studied.

2. EXPERIMENTAL SECTION 2.1. Materials. Bituminous coal and coking coal were used as carbon precursor to prepare activated coke in this study. Bituminous coal mainly contained (wt) 73.8% C, 9.7% H, 14.1% O, 2.3% N, and 0.1% S, and coking coal mainly consisted of (wt) 74.2% C, 4.8% H, 17.4% O, 2.1% N, and 1.5% S. The two kinds of raw coals were obtained from Shanxi Province, China. Copper ore was obtained from Jiangxi Province of China and primarily consisted of (wt): 25.0% Cu, 29.0% Fe, 31.9% S, 3.4% Si, 0.4% Ca, and 0.1% Zn. All the raw materials were grinded, sieved, and dried before use. 2.2. Preparation of AC. The samples were prepared by blending and via a simple one-step carbonization−activation process. In a typical preparation process, 70.0 g of bituminous coal and 30.0 g of coking coal powder were first mixed thoroughly in a mixer. Copper ore powder was directly blended with the coals mixture at the weight ratio of 1 to 12% and then mixed in a kneading machine. Subsequently, distilled water and coal tar were added as the binder and the mixture was continuously stirred at 70 °C. Next, the resultant mixture was molded to columnar shape with the diameter of 3.0 mm in a vacuum hydraulic extruder at 10 MPa pressure. After that, the columnar material was carbonized at 600 °C under nitrogenatmosphere for 60 min and then activated at 900 °C by certain water steam (MC:MH2O= 2) for 40 min in a rotary furnace. The heating rate in the overall process was 5 °C/min. The as-prepared samples were designated as “AC-Copn”, where “n” represented the weight ratio of copper ore to the coals mixture. For comparison, a control sample was prepared without the addition of copper ore and named as “AC”. 2.3. Characterization. The element of two raw coals was measured by elemental vario EL cube (Elementar, Germany).

3. RESULTS AND DISCUSSION 3.1. Structure and Chemical Properties of AC-Cop. 3.1.1. Surface Morphology and Porous Properties. A typical SEM picture of AC-Cop1 is presented in Figure 1a, which showed a rough surface with some fine crystals. The EDAX 15732

DOI: 10.1021/acs.iecr.8b03872 Ind. Eng. Chem. Res. 2018, 57, 15731−15739

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Industrial & Engineering Chemistry Research

High-resolution XPS was applied to further investigate the functional groups of prepared samples (SI, Figure S3). The high-resolution C 1s spectra of all samples were curve-fitted into four peaks with binding energies of 284.8 ± 0.2, 285.8 ± 0.2, 287.3 ± 0.2, and 289.4 ± 0.3 eV, which corresponded to C−C, C−O, CO, and π−π* groups, respectively.18,19 Table 2 summarizes the relative contents of functional groups

images (Figure 1b,c) further indicated that Cu and Fe were homogeneously distributed on the surface of AC rather than forming agglomerations deposited on the surface or filled in the pores. This suggested that the addition of copper ore into AC would not cause the blockage of pores. As shown in Supporting Information (SI), Figure S1, all the prepared samples have type-I isotherms with obvious adsorption at P/P0 < 0.1, demonstrating that the obtained ACs were microporous materials. The detailed texture properties of the ACs calculated from N2 adsorption− desorption isotherms are summarized in Table 1, with the

Table 2. Relative Contents of C 1s of the Prepared Samples relative content (%)

Table 1. Textural Properties and Sulfur Capacities of the Prepared Samplesa sample AC AC-Cop1 AC-Cop3 AC-Cop6 AC-Cop9 AC-Cop12

SBET Vtot Vmic Vmeso (m2 g−1) (cm3 g−1) (cm3 g−1) (cm3 g−1) 442 408 400 418 419 403

0.240 0.205 0.213 0.234 0.232 0.215

0.181 0.183 0.166 0.171 0.172 0.175

0.042 0.010 0.047 0.048 0.037 0.040

sulfur capacity (mg g−1) 120.1 174.8 129.4 135.2 144.2 152.6

sample

C−C

C−O

CO

π−π*

AC AC-Cop1 AC-Cop3 AC-Cop6 AC-Cop9 AC-Cop12

65.98 69.58 66.66 66.84 67.50 66.82

22.7 17.32 19.64 18.16 17.94 17.60

6.54 8.67 8.07 8.82 7.71 6.99

4.78 4.43 5.63 6.18 6.85 8.59

calculated from the high-resolution XPS spectra. After copper ore modification, the relative contents of C−C group did not significantly change, compared with the untreated one. The relative contents of CO group increased evidently, while the opposite trend for the C−O group was observed. This demonstrated that after copper ore modification the surface of ACs became more basicity as a result of the Brønsted basic properties of CO group and the acidic characteristics of C− O group.20,21 The basicity surface is conductive to the adsorption of SO2 because of the acidic property of SO2 gas. 3.1.3. Metal Phase. XRD analysis was conducted on the prepared ACs, as shown in Figure 3a. For all the prepared samples, the peaks of SiO2 at 2θ = 20.8° and 26.8° (JCPDs 511593) were observed, which are developed from the coals used in the samples preparation process. After the modification by copper ore, the characteristic peaks of CuO at 2θ = 28.8° and 34.5° (JCPDs 45-0937) were detected in all AC-Cop samples. Meanwhile, there were also the characteristic peaks of Fe2O3 at 2θ = 45.1°, 47.5°, 56.4°, and 64.9° (JCPDs 39-0238) and Fe3O4 at 2θ = 31.3° and 36.6° (JCPDs 26-1136) in AC-Cop samples. Moreover, the intensities of the peaks of CuO and Fe2O3 increased with the increasing blend ratio of copper ore. However, for copper ore, there was only FeS at 2θ = 77.9° (JCPDs 23-1123) and CuFeS2 at 2θ = 29.5°, 48.8°, and 57.9° (JCPDs 37-0471), while no discernible peaks corresponded to

a

SBET, BET surface areas; Vtot, total pore volume; Vmic, micropore volume; Vmeso, mesopore volume.

SBET ranging from 403 to 442 m2/g, Vtot ranging from 0.21 to 0.24 cm3/g, and Vmic ranging from 0.16 to 0.18 cm3/g. The pore size distribution for the prepared samples was similar (SI, Figure S2), with the range of average pore size from 2.13−2.33 nm, suggesting that the weight ratio of copper ore to coals mixture had not an obvious effect on the pore size distribution over the modified AC. All the prepared ACs had the similar textural properties, indicating that the modification by copper ore did not significantly change the pore structure of ACs. 3.1.2. Surface Functional Groups. The FTIR spectra of ACs are shown in Figure 2a. The broad bands at around 3450, 1630, and 1080 cm−1 could be observed in all the ACs, which are identified as O−H, CO, and C−O groups vibration, respectively.5,14 Moreover, the intensities of the O−H, CO, and C−O groups of copper ore modified samples were stronger than that of blank one. The result indicated that the oxygen-containing functional groups may be introduced by copper ore modification during preparation process.

Figure 2. FTIR spectra of the prepared samples before (a) and after (b) desulfurization. 15733

DOI: 10.1021/acs.iecr.8b03872 Ind. Eng. Chem. Res. 2018, 57, 15731−15739

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Industrial & Engineering Chemistry Research

Figure 3. XRD spectra of the prepared samples before (a) and after (b) desulfurization.

CuO, Fe2O3, and Fe3O4 were found. The results indicated that there could be strong interactions between copper ore and carbon matrix during AC-Cop preparation process. The XPS high-resolution analysis of Cu and Fe of AC-Cop are displayed in SI, Figure S4. For all modified samples, Cu 2p spectra were composed of three main peaks at 935.4 ± 0.1, 945.2 ± 0.2, and 955.6 ± 0.3 eV, which are due to the characteristics of Cu2+ in CuO.22,23 This demonstrated that CuO was the major Cu species in all AC-Cop, in accordance with XRD analysis (Figure 3a). For the fitting peaks of Fe 2p of AC-Cop, around 712 and 725 eV were assigned to Fe 2p3/2 and Fe 2p1/2 peaks, respectively, which are ascribed to Fe3+ in Fe2O3.7,24 However, no clear XPS spectra for Fe3O4 were detected on all AC-Cop. This might occur as a result of the sensitivity of Fe3O4 on AC that can be oxidized easily to Fe2O3 during the sample preparation process for XPS analysis.7,24 This result could also be partly due to the surface specificity of XPS measurements which only estimates the chemical composition of outermost surface layer (less than 5 nm in depth).23 According to the results of XRD and XPS analysis for copper ore and the AC-Cop, we can assume that there were some strong interactions between copper ore and carbon matrix during AC-Cop preparation process, resulting in the disappearance of CuFeS2 and FeS and the formation of CuO, Fe2O3, and Fe3O4 on AC-Cop (Figure 3a).When the mixture of copper ore and coals was activated with water vapor, a series of redox reactions would occur, which could be assumed as follows:10,25,26 C + H 2O = H 2↑ + CO↑

Figure 4. SO2 breakthrough curves of prepared samples.

of blank AC. Moreover, the value was also significantly higher than that of commercial AC (104 mg g−1).13 The high sulfur capacity of AC-Cop1 could be related to the high content of CO groups on the surface of AC-Cop1, which is more suitable for SO2 adsorption. Meanwhile, the higher content of Fe3O4 on AC-Cop1 (Figure 3) would also exhibit the higher catalytic activity toward SO2 than that of Fe2O3.7,27 On the other hand, the sulfur capacity of AC-Cop increased with the increase of the copper ore ratio when their ratio was higher than 3%, which may be related to the increased content of Fe2O3 (Figure 3). The results demonstrated that it would be feasible to transform common activated coke into a superior desulfurization catalyst, which could be achieved via the addition of a low content of copper ore only. It is worth noting that the AC-Cop and blank AC had almost similar porous structure including SBET and Vmic, while their desulfurization activities were obviously different (Table 1). It was reported that both SBET and Vmic were the key factors for the adsorption of SO2.28 This result indicated that porous texture might be not the critical factor for influencing the desulfurization activity of AC-Cop. It is possible that porous structure of the samples had met the needs of molecular diffusion for desulfurization. On the other hand, the chemical characteristics of AC surface could play important roles in the removal of SO2, including some active components, i.e., metal

(1)

2CuFeS2 + 5H 2O = Fe2O3 + 2CuO + 4H 2S↑ + H 2↑ (2)

3Fe2O3 + CO = 2Fe3O4 + CO2 ↑

(3)

3.2. Desulfurization Performance of AC-Cop. Figure 4 displays the SO2 breakthrough curves of the prepared samples. The outlet concentrations of SO2 for all ACs gradually increased with desulfurization time, suggesting a gradual deactivation process. From the breakthrough curves, the sulfur capacities are presented in Table 1. Especially, the AC-Cop showed better desulfurization activity than AC, indicating that the modification by copper ore could significantly improve the desulfurization activity of AC. The sulfur capacity of AC-Cop1 was the highest at 174.8 mg/g, which was 50% higher than that 15734

DOI: 10.1021/acs.iecr.8b03872 Ind. Eng. Chem. Res. 2018, 57, 15731−15739

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Industrial & Engineering Chemistry Research

Figure 5. XPS survey of AC-Cop1 before and after desulfurization test (a) and high-resolution S 2p spectra of AC-Cop1 after desulfurization test (b).

deposition of desulfurization products on AC, i.e., adsorbed SO2, H2SO4, Fe2(SO4)3, FeSO4, and CuSO4, etc.29 In this study, thermal regeneration was applied to remove desulfurization products for recycling sulfur resource and meanwhile recover the desulfurization activity of used AC. The sulfur capacities of AC-Cop1 after thermal regeneration are displayed in Table 3. During four desulfurization−regeneration runs,

oxides (CuO, Fe2O3, and Fe3O4) and basic functional groups (CO).8,14,29 3.3. Characterization of AC-Cop after Desulfurization. It is significantly important to understand the effects of chemical characteristics of AC-Cop on the adsorption and oxidation of SO2 because they played important roles in the removal of SO2. Figure 2b shows the FTIR spectra of ACs after desulfurization. For all samples, the intensities of CO group decreased after desulfurization, while those of C−O and O−H groups increased significantly. This indicated that the oxygencontaining groups were involved in the removal of SO2. Meanwhile, a new peak at 595 cm−1 was found for all the used AC, which belongs to the stretching of sulfur in sulfate,30 demonstrating that SO2 can be oxidized to SO42− on samples. Furthermore, the intensities of S−O or SO of AC-Cop were stronger than that of blank AC, indicating that the catalytic oxidation effect of AC-Cop on SO2 was stronger than that of blank AC. This can be attributed to the more active sites on AC-Cop, i.e., CuO, Fe2O3, Fe3O4, and CO groups. The XPS analysis was also conducted on the used AC-Cop, as shown in Figure 5. After desulfurization, a clearly new peak at 169.8 eV was observed on the surface of used AC-Cop1. From the high-resolution spectra S 2p of used AC-Cop1 (Figure 5b), the peak was assigned to sulfur in the form of sulfate (SO42−).31 This was consistent with the results of FTIR (Figure 2b). The high-resolution spectra of Fe 2p and Cu 2p did not change after desulfurization (Figure S5), indicating that the state of Fe and Cu on AC-Cop did not change during desulfurization process. The metal phase on the used AC was further analyzed by XRD, and the results are shown in Figure 3b. After desulfurization, no discernible peaks corresponded to CuO, Fe2O3, and Fe3O4 were observed, while there were several clear new peaks, which were assigned to Fe2(SO4)3 at 25.5° and 48.9° (JCPDs 47-1774), FeSO4 at 36.6° (JCPDs 22-0357), and CuSO4 at 26.8°, 31.3°, and 43.1° (JCPDs 11-0646). This indicated that CuO, Fe2O3, and Fe3O4 on AC-Cop were involved in SO2 removal process and transformed into corresponding metal sulfate, i.e., CuSO4, Fe2(SO4)3, and FeSO4. Thus, it is certain that CuO, Fe2O3, and Fe3O4 on the surface of AC-Cop were the active components for the removal of SO2. The generated metal sulfates could deposit on the surface of AC, and the low content of metal irons may form a new liquid-phase catalyst for SO2 oxidation.12 3.4. Regeneration of Used AC-Cop. As shown in Figure 4, all the prepared samples exhibited a gradual deactivation process for the removal of SO2, which can be attributed to the

Table 3. Textural Properties and Sulfur Capacities of ACCop1 and Regenerated AC-Cop1 sample AC-Cop1 AC-Cop1-R1 AC-Cop1-R2 AC-Cop1-R3 AC-Cop1-R4

SBET Vtot Vmic Vmeso (m2 g−1) (cm3 g−1) (cm3 g−1) (cm3 g−1) 408 505 475 456 428

0.205 0.300 0.267 0.256 0.255

0.183 0.196 0.189 0.183 0.165

0.010 0.084 0.059 0.056 0.062

sulfur capacity (mg g−1) 174.8 131.6 115.2 102.8 92.5

a

SBET, BET surface areas; Vtot, total pore volume; Vmic, micropore volume; Vmeso, mesopore volume.

there was a slow decrease in desulfurization activity of ACCop1, and the sulfur capacity was 92.5 mg/g at the fourth test. The value was still close to the sulfur capacity of commercial activated coke (104 mg g−1).13 Meanwhile, such value was also higher than that of the regenerated blank AC, which was only 52.6 mg g−1 at the fourth run. This result showed that copper ore modified AC was a kind of regenerable catalyst for the removal of SO2. Porous structure of the fresh and regenerated AC-Cop1 was compared. As shown in SI, Figure S6, the nitrogen adsorption−desorption isotherms of all the regenerated ACCop1 were the typical type IV with obvious hysteresis loops at 0.5−0.9 P/P0, indicating the existence of mesopores. The detailed pore structure data are summarized in Table 3. The Vmic of regenerated AC-Cop1 did not obviously change compared with fresh sample, while the SBET, Vtot, and Vmeso increased. There were a large number of mesopores for ACCop1 after the thermal regeneration, which could be primarily attributed to the chemical reactions between carbon matrix and the desulfurization product H2SO4:16 C + H 2SO4 = CO + SO2 + H 2O

(4)

C + 2H 2SO4 = CO2 + 2SO2 + 2H 2O

(5) 3+

2+

Meanwhile, the metal ions in sulfates, i.e., Fe , Fe , and Cu2+, had the catalytic effect to induce the development of 15735

DOI: 10.1021/acs.iecr.8b03872 Ind. Eng. Chem. Res. 2018, 57, 15731−15739

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Industrial & Engineering Chemistry Research

Figure 6. XPS survey (a) and XRD spectra of AC-Cop1 and AC-Cop1-R4 (b).

mesopore under thermal treatment.32,33 In addition, the results further indicated that porous texture is not the critical factor for influencing the desulfurization activity of AC-Cop, while some active components on the surface of AC-Cop might play much more important roles in desulfurization such as functional groups and transition metal oxides. The chemical properties of regenerated AC-Cop1 after four desulfurization−regeneration tests were analyzed by XPS and XRD. As shown in Figure 6a, a clear peak belonging to sulfur at 169.8 eV was also found after four times of desulfurization− regeneration.6 This demonstrated that the desulfurization products could gradually be accumulated on the surface of the catalyst during desulfurization−regeneration cycles. Highresolution XPS spectra of S 2p (SI, Figure S7) further indicated that the sulfur species on the surface of AC-Cop-R4 was in the form of SO42−. The results of XRD analysis for ACCop1-R4 (Figure 6b) showed that the sulfur species on the AC-cop1-R4 mainly existed as Fe2(SO4)3 and CuSO4. This indicated that the metal sulfates still deposited on the surface of the catalyst after thermal regeneration, which should be responsible for the decrease of desulfurization performance. However, no obvious peaks corresponded to FeSO4 were observed for AC-Cop1-R4. This could be because Fe2+ in FeSO4 is so sensitive to oxygen that FeSO4 on the surface of AC-Cop can be oxidized to Fe 2(SO 4 )3 easily during regeneration process. As shown in Table 4, the contents of CO group decreased gradually from 9.17 to 6.61% after four desulfurization− regeneration tests, while those of C−O group increased from 16.82 to 23.41%. The change of surface functional groups contents was not beneficial to remove acidic gas such as SO2

due to the Brønsted basic properties of CO group and acidic characteristics of C−O group.34,35 3.5. Possible Reaction Mechanism of the SO 2 Removal on AC-Cop. It was obvious that the addition of copper ore significantly promoted the adsorption and oxidation of SO2, resulting in the high desulfurization performance of AC-Cop (Figure 4). This could be attributed to the introduction of some active components on AC. On the basis of our observed results, it is possible to assume the primary desulfurization mechanisms of AC-Cop, as shown in Figure 7, and there could be three kinds of desulfurization active components: (1) oxygen-containing functional group (i.e., CO) on carbon matrix,; (2) CuO on AC-Cop, and (3) Fe2O3 and Fe3O4 on AC-Cop. The following reactions have been described as the representative for SO2 bonding to carbon matrix:27,28,36 SO2 (g) → SO2 (ads)

(6)

SO2 (ads) + O(ads) → SO3(ads)

(7)

CO groups on carbon matrix could increase the basicity of AC, which would help to facilitate the adsorption of SO2.29,35 Meanwhile, the SO3 formed could be adsorbed by CO groups on the surface of carbon matrix (eq 8),29,37

Furthermore, H2SO4 would be produced as the result of H2O vapor in flue gas (eq 9),

Table 4. Relative Contents of C 1s of AC-Cop1 and Regenerated AC-Cop1 relative content (%) sample

C−C

C−O

CO

π−π*

AC-Cop1 AC-Cop1-R1 AC-Cop1-R2 AC-Cop1-R3 AC-Cop1-R4

69.58 68.04 67.44 66.38 64.16

16.82 17.73 19.44 21.24 23.41

9.17 8.43 7.26 6.94 6.61

4.43 5.80 5.86 5.44 5.82

The formation of H2SO4 could be responsible for the consumption of the CO group during desulfurization process. In addition, some of the generated H2SO4 was washed off by water film, while the rest was retained on the surface and in the pores of AC, which could increase the surface acidity and then cause a decrease in SO2 adsorption. 15736

DOI: 10.1021/acs.iecr.8b03872 Ind. Eng. Chem. Res. 2018, 57, 15731−15739

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Industrial & Engineering Chemistry Research

Figure 7. Possible reaction mechanism of SO2 removal on AC-Cop.

the copper ore-modified AC could overcome the limitation of traditional AC desulfurizers and provide an efficient, regenerable, and inexpensive solution for the removal of SO2.

SO2 can be catalytically oxidized to SO3 on CuO in the presence of O2 and then reacted with the copper sites to form sulfates connected to copper (eq 10).8,17 SO2 +

CuO CuO 1 O2 ⎯⎯⎯→ SO3 ⎯⎯⎯→ CuSO4 2

■

(10)

It has been proved that the gaseous SO2 cannot react with iron oxides in the air directly.38,39 However, SO2 can easily connect to iron oxides (Fe2O3 and Fe3O4) to produce H2SO3 when there is water vapor.40 The subsequent reactions between H2SO3 and adsorbed O2 resulted in the production of H2SO4.39,41 H2SO4 reacted with iron oxides (Fe2O3 and Fe3O4) into iron sulfates (FeSO4 and Fe2(SO4)3) (Figure 3b). The corresponding reactions can be assumed as follows: SO2 +

Fe3O4 /Fe2O3 1 O2 + H 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H 2SO4 2

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b03872. N2 adsorption−desorption isotherms of the prepared samples; pore size distributions for prepared samples; XPS spectra of C 1s patterns for the prepared samples; XPS spectra of Fe 2p and Cu 2p patterns for the prepared samples. XPS spectra of Fe 2p and Cu 2p for AC-Cop1 before and after desulfurization; N2 adsorption−desorption isotherms of AC-Cop1 and regenerated AC-Cop1.XPS survey and high-resolution spectra of S 2p, Cu 2p, and Fe 2p for AC-Cop-R4 (PDF)

(11)

4H 2SO4 + Fe3O4 → FeSO4 + Fe2(SO4 )3 + 4H 2O

(12)

3H 2SO4 + Fe2O3 → Fe2(SO4 )3 + 3H 2O

(13)

ASSOCIATED CONTENT

S Supporting Information *

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-28-85467800. Fax: +86-28-85405613. E-mail: [email protected].

The metal sulfates produced during desulfurization process, i.e., CuSO4, FeSO4, and Fe2(SO4)3, would gradually accumulate on AC-Cop, resulting in a slow deactivation process through covering the active sites.

ORCID

Xia Jiang: 0000-0001-5754-7937 Wenju Jiang: 0000-0003-1327-7159

4. CONCLUSIONS The results show that a novel AC-Cop could be prepared by blending method via a simple one-step carbonization− activation process for flue gas desulfurization. CuO, Fe2O3, and Fe3O4 were detected on the surface of AC-Cop due to the chemical reactions between copper ore and coals during carbonization−activation process. Meanwhile, their contents of CO groups were significantly increased in comparison with blank one. As a result, the high sulfur capacity of AC-Cop1 was achieved at 174.8 mg/g, which was 50% higher than that of AC. Furthermore, we found that the surface chemical properties of the AC-Cop played a more important role than porous structure in the removal of SO2, with the active components including CuO, Fe2O3, Fe3O4, and CO groups. The AC-Cop can be easily regenerated by thermal treatment, with a slow decrease in desulfurization performance during four regeneration tests. The desulfurization results suggest that

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was founded by National Nature Science Foundation of China (no. 51778383) and Science & Technology Department of Sichuan Province (2018HH0096).

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REFERENCES

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DOI: 10.1021/acs.iecr.8b03872 Ind. Eng. Chem. Res. 2018, 57, 15731−15739